Apoptosis, a form of programmed cell death, typically occurs in the normal development and maintenance of healthy tissues in multicellular organisms. It is a complex process, which results in the removal of damaged, diseased or developmentally redundant cells, without signs of inflammation or necrosis. Apoptosis thus occurs as a normal part of development, the maintenance of normal cellular homeostasis, or as a consequence of stimuli such as chemotherapy and radiation.
The intrinsic apoptotic pathway is known to be deregulated in cancer and lymphoproliferative syndromes, as well as autoimmune disorders such as multiple sclerosis and rheumatoid arthritis. Additionally, alterations in a host apoptotic response have been described in the development or maintenance of viral and bacterial infections. Cancer cells gain the ability to overcome or circumvent apoptosis and continue with inappropriate proliferation despite strong pro-apoptotic signals such as hypoxia, endogenous cytokines, radiation treatments and chemotherapy. In autoimmune disease, pathogenic effector cells can become resistant to normal apoptotic cues. Resistance can be caused by numerous mechanisms, including alterations in the apoptotic machinery due to increased activity of anti-apoptotic pathways or expression of anti-apoptotic genes. Thus, approaches that reduce the threshold of apoptotic induction in cancer cells by overcoming resistance mechanisms may be of significant clinical utility.
Caspases serve as key effector molecules in apoptosis signaling. Caspases (cysteine containing aspartate specific proteases) are strong proteases and once activated, digest vital cell proteins from within the cell. Since caspases are highly active proteases, tight control of this family of proteins is necessary to prevent premature cell death. In general, caspases are synthesized as largely inactive zymogens that require proteolytic processing for activation. This proteolytic processing is only one of the ways in which caspases are regulated. The second mechanism of regulation is through a family of proteins that bind and inhibit caspases.
One family of molecules that inhibit caspases are the Inhibitors of Apoptosis (IAP) (Deveraux et al., J Clin Immunol (1999), 19: 388-398). IAPs were originally discovered in baculovirus by their ability to substitute for P35 protein function, an anti-apoptotic gene (Crook et al. (1993) J Virology 67, 2168-2174). Human IAPs are characterized by the presence of one to three homologous structural domains known as baculovirus IAP repeat (BIR) domains. Some IAP family members also contain a RING zinc finger domain at the C-terminus, with the capability to ubiquitylate target proteins via their E3 ligase function. The human IAPs, XIAP, HIAP1 (also referred to as cIAP2), and HIAP2 (cIAP1) each have three BIR domains, and a carboxy terminal RING zinc finger. Another IAP, NAIP, has three BIR domains (BIR1, BIR2 and BIR3), but no RING domain, whereas Livin, TsIAP and MLIAP have a single BIR domain and a RING domain. The X chromosome-linked inhibitor of apoptosis (XIAP) is an example of an IAP, which can inhibit the initiator caspase Caspase-9, and the effector caspases, Caspase-3 and Caspase-7, by direct binding. XIAP can also induce the degradation of caspases through the ubiquitylation-mediated proteasome pathway via the E3 ligase activity of a RING zinc finger domain. Inhibition of Caspase-9 is mediated by the BIR3 domains of XIAP, whereas effector caspases are inhibited by binding to the linker-BIR2 domain. The BIR domains also mediate the interactions of IAPs with tumor necrosis factor-receptor associated factor (TRAFs)-I and -2, and with TAB1, adaptor proteins affecting survival signaling through NFkB activation. IAP proteins can thus function as direct brakes on the apoptosis cascade by inhibiting active caspases or by redirecting cellular signaling to a pro-survival mode. Survivin is another member of the IAP family of antiapoptotic proteins. It is shown to be conserved in function across evolution as homologues of the protein are found both in vertebrates and invertebrates.
Cancer cells and cells involved in autoimmune disease may avoid apoptosis by the sustained over-expression of one or more members of the IAP family of proteins. For example, IAP overexpression has been demonstrated to be prognostic of poor clinical outcome in multiple cancers, and decreased IAP expression through RNAi strategies sensitizes tumor cells to a wide variety of apoptotic insults including chemotherapy, radiotherapy and death receptor ligands. For XIAP, this is shown in cancers as diverse as leukemia and ovarian cancer. Over expression of cIAP1 and cIAP2 resulting from the frequent chromosome amplification of the 11q21-q23 region, which encompasses both genes, has been observed in a variety of malignancies, including medulloblastomas, renal cell carcinomas, glioblastomas, and gastric carcinomas.
The interaction between the baculoviral IAP repeat-3 (BIR3) domain of X-linked inhibitor of apoptosis (XIAP) and caspase-9 is of therapeutic interest because this interaction is inhibited by the NH2-terminal seven-amino-acid residues of the so-called “second mitochondrial-derived activator of caspase” (in short and hereinafter Smac), a naturally occurring antagonist of IAPs. Small-molecule Smac mimetics have been generated anticipating efficacy in cancer by reconstituting apoptotic signaling.
Thus, there is the need to provide SMAC mimetics useful for the prevention and/or treatment of diseases characterized by excessive or abnormal cell proliferation, such as cancer.